1. Field of the Invention
The present invention relates to a method for determining at least one time constant of a reference model in a cascaded controlling arrangement.
2. Description of the Related Art
Usually a cascaded controlling structure, including a position, rpm and current control device, is employed in numerically controlled machine tools. As a rule, the speed control device, which is connected downstream of the position control device, is embodied as a PI speed control device and includes a proportional branch (P) and an integral branch (I). The phase response of the upstream connected position control device worsens as a result of the effect of the integral branch of the speed control device. It is therefore necessary as a consequence of this to reduce the loop gain kV of the position control device a priori in order to prevent oscillations in the drive systems of the machine tool controlled by the controlling device. However, as large as possible a loop gain kV of the position control device is desired in principle.
To solve these problems, it has already been suggested by P. Ernst and G. Heinemann in the course of a seminar presentation under the title “Optimierte Achsregelung mit durchgängig offenen CNC-Steuerungen” [Optimized Axis Control with Continuously Open CNC Controls] (ISW Position Controlling Seminar 1999, 26, Mar. 27, 1999) in Chapter 2.2 to connect a reference model upstream of the speed control device. The reference model, designed as a 2nd order time-delay element, is matched to the behavior of the closed speed control device without an integral portion. It is possible in this way to eliminate, or at least to minimize, the detrimental influence of the integral portion on the control behavior of the speed control device. However, the desired elimination of disturbances without integral portions continues to be fully maintained. However, no further suggestions can be found in the cited reference regarding suitable parameterization, in particular the determination of suitable time constants, of a corresponding 2nd order reference model.